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Re: password file locking

ok, you should _Definitely_ examine lib/util_file.c in the samba source,
and how it is used to update smbpasswd.

i believe that the problems we faced and solved with smbpasswd are exactly
those you are describing here.

On Wed, 30 Aug 2000, Cooper wrote:

> Michael Tokarev wrote:
> > 
> > But wait -- one big issue with this exists.  Note that
> > "dotlock" file created before opening real /etc/passwd file,
> > and it does _not_ removed after use. But applications that
> > updates /etc/passwd need to have reliable way to change
> > contents of that file, that is typically implemented by
> > creating new file and issuing two rename()s.
> I feel I'm starting to lose track here, so am I right in assuming you
> rename passwd to something else (passwd.old) and then the new thing to
> the new passwd?
> > With this,
> > it is hard to ensure that second app that waits for a lock
> > will use correct /etc/passwd when first lock released, not
> > a file that was renamed by first app.
> Correct. Which I think is exactly why you shouldn't use a second file
> when you try to lock a file. When you rename a new file to the current
> name, anybody who has the current file open will basically be using old
> data once the rename is complete. If that old data was used as a
> starting point for a modification the fun can begin.

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